A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the manufacture of devices using lithographic processes, each mask pattern is typically projected onto the target portion in focus. In practice, this means that the target portion of the substrate is positioned in a plane of best focus of the aerial image projected by the projection system. As the critical dimension (CD), i.e. the dimension of a feature or features in which variations will cause undesirable variation in physical properties of the feature, such as the gate width of a transistor, in lithography shrinks, consistency of focus, both across a substrate and between substrates, becomes increasingly important. Traditionally, lithography apparatus have used an image sensor to probe the aerial image or optimal settings were determined by “send-ahead wafers”, i.e. substrates that are exposed, developed and measured in advance of a production run. In the send-ahead wafers, test structures are exposed in a so-called focus-energy matrix (FEM) and best focus and energy settings were determined from examination of those test structures.
The use of an alignment system to monitor focus has been proposed and involves printing focus-sensitive alignment markers at known positions relative to normal alignment markers at various different focus settings, i.e. positions of the substrate relative to the projection system. The position of these focus-sensitive markers with respect to the normal alignment markers is measured and an alignment offset (AO) shows up which is representative of focus errors.
However, this method may use valuable machine time, both to print the markers and to make the alignment measurements necessary to determine the alignment offsets. Also, the method may use markers with a period matched to the alignment sensor, e.g. approximately 16 μm, which is considerably larger than the period of critical structures in a device pattern. Thus, focus settings determined as optimum for the alignment markers may not also be optimum for the device structures.